For the formation of a coating film on the bodies of vehicles such as automobiles, motorcycles, and the like, one widely used method comprises applying a thermosetting coating composition that contains a resin having thermosetting functional groups, such as a hydroxyl-containing acrylic resin, and a curing agent, such as a melamine resin, to a vehicle body to be coated, and thereafter curing the applied coating composition by heating. This method can form a coating film having excellent film properties in terms of gasoline resistance, adhesion, etc.
In recent years, it has been desired to reduce energy consumption and increase productivity in the above-mentioned coating film formation method. For example, the heat-curing step on an automobile body coating line is usually conducted at about 140° C. for about 40 minutes; when the conveyor runs at a speed of 3 m/min, the step requires a space for the drying furnace line about 120 m in length. Therefore, in order to save space and energy, shortening of the heating time in the heat-curing step is demanded. Moreover, reduction of the heating time is preferable so as to decrease CO2, soot, etc., which cause environmental problems.
In addition, in view of better appearance, the coating film formed on an automobile body is now increasingly required to have improved resistance to marring by car washes, scratching around keyholes, etc. It has thus become important to develop a coating composition that can form a coating film having excellent resistance to marring or scratching on an automobile body or the like.
Use of coating compositions that are curable by active energy rays such as ultraviolet rays is effective for reducing or omitting heating steps. For example, Japanese Unexamined Patent Publications No. 1999-124403 and No. 1999-124404 disclose an active energy ray-curable composition comprising a maleimide derivative that can be cured under ultraviolet irradiation. However, although coating with such a composition can shorten the time to cure a coating film or can reduce the size of coating facilities, the curing of the coating film is insufficient, so that it is difficult to obtain a coating film having excellent properties in mar resistance, scratch resistance, gasoline resistance, adhesion, etc.
Japanese Unexamined Patent Publications No. 2001-220536 and No. 2002-320910 disclose an active energy ray-curable coating composition comprising an acrylic resin wherein the acrylic resin is a copolymer of a (meth)acrylic acid alkyl ester, a (meth)acrylic acid hydroxyalkyl ester, and a monomer having an unsaturated double bond and a maleimide group; and a coating method using this coating composition. However, although such a coating composition or coating method can shorten the time to cure a coating film or can reduce the size of coating facilities, the curing of the coating film is insufficient. As a result, no coating films can be obtained that are suitable for the bodies of automobiles, etc., having excellent properties in mar resistance, scratch resistance, gasoline resistance, adhesion, etc.